Machine tool with fault detection

Abstract

A machine tool includes a spindle having a cutting tool for cutting a workpiece. For fault detection, a control unit of the machine tool detects a value representing a torque applied on the spindle while the spindle is stopped in a state the cutting tool is engaged with the workpiece. The control unit further compares the detected value (representing a torque) with a predetermined threshold thereby to determine that a fault has occurs. Since the detection of the value representing a torque is performed while the spindle is stopped, the detection is free from the effects of acceleration or deceleration of the spindle.

What is claimed is: 
     
       1. A machine tool comprising: a cutting tool;   a spindle which holds said cutting tool;   a drive unit arranged to drive said spindle so that said cutting tool cuts a workpiece and to drive said spindle in a reverse direction so that said cutting tool retracts from said workpiece; and   a control unit arranged to detect a value representing a torque applied on said spindle and to compare said value with a predetermined threshold thereby to determine if a fault has occurred;   wherein said control unit detects said value when said drive unit stops said spindle in a state that said cutting tool is acting on said workpiece with applying the torque on said spindle to keep the engagement of the cutting tool with said workpiece.   
     
     
       2. The machine tool according to claim 1, wherein said drive unit rotates and feeds said spindle respectively in a first rotational direction and a first feeding direction, thereby to cut said workpiece with said cutting tool. 
     
     
       3. The machine tool according to claim 2, wherein said drive unit rotates and feeds said spindle respectively in a second rotational direction and a second feeding direction, thereby to retract said cutting tool from said workpiece. 
     
     
       4. The machine tool according to claim 3, wherein said drive unit includes a rotation motor driven to rotate said spindle and a feeding motor driven to feed said spindle. 
     
     
       5. The machine tool according to claim 4, wherein said value representing a torque is a value of a drive current sent from said control unit to said rotation motor. 
     
     
       6. The machine tool according to claim 5, wherein said rotation motor is a servo-motor and said control unit further comprises a servo-amplifier, said value being a drive current from said servo-amplifier to said rotation motor. 
     
     
       7. The machine tool according to claim 3, wherein, if said control unit detects the occurrence of a fault, said drive unit rotates and feeds said spindle respectively in said second rotational direction and said second feeling direction at lower speeds. 
     
     
       8. The machine tool according to claim 1, further comprising a signal generating system which generates a signal when said control unit detects the occurrence of a fault. 
     
     
       9. The machine tool according to claim 1, wherein said control unit is arranged to classify faults according to said detected value. 
     
     
       10. The machine tool according to claim 9, wherein said control unit is arranged to select a retracting speed, based on the classification of a fault. 
     
     
       11. The machine tool according to claim 10, wherein faults are classified into a plurality of classes of faults including a first-class-fault and a second-class-fault, and wherein said first-class-fault is set such that the occurrence of said first-class-fault increases said torque, and wherein said second-class-fault is set such that the occurrence of said second-class-fault decreases said torque.   
     
     
       12. The machine tool according to claim 11, wherein said control unit has a plurality of thresholds including a first threshold and a second threshold respectively used to discriminate said first-class-fault and said second-class-fault from a state in which no fault occurs. 
     
     
       13. The machine tool according to claim 12, wherein said control unit determines the occurrence of said first-class-fault if said value is larger than said first threshold and determines the occurrence of said second-class-fault if said value is smaller than said second threshold. 
     
     
       14. The machine tool according to claim 11, wherein said first-class-fault is further classified based on said value into at least two classes. 
     
     
       15. The machine tool according to claim 14, said control unit having at least one additional threshold used to discriminate said at least two classes of said first-class-fault. 
     
     
       16. The machine tool according to claim 1, further comprising a stop detecting system which detects if said rotation is stopped. 
     
     
       17. The machine tool according to claim 16, wherein said stop detecting system is arranged to check if said rotation is stopped, based on detecting one of the rotation speed and the feeding speed of said spindle. 
     
     
       18. The machine tool according to claim 16, wherein said stop detecting system is arranged to check if said rotation is stopped, based on detecting the rotation position of said spindle. 
     
     
       19. The machine tool according to claim 1, wherein said cutting tool is a tap used to cut an internal thread. 
     
     
       20. A method for detecting a fault on a cutting tool during machining a workpiece with said cutting tool, said cutting tool being provided on a spindle, said method comprising the steps of: rotating said spindle with a torque required to resist a reactive force from said workpiece, so that said cutting tool cuts said workpiece;   stopping said cutting tool in a state that said cutting tool is engaged with said workpiece, while keeping applying said torque on said spindle;   reading a value representing said torque applied on said cutting tool;   comparing said value with at least one predetermined threshold; and   determining if a fault has occurred, based on a result of said comparing step.   
     
     
       21. The method according to claim 20, further comprising a step of retracting said cutting tool from said workpiece after said comparing step. 
     
     
       22. The method according to claim 21, wherein, if said control unit detects the occurrence of a fault, said retracting step is performed at a lower speed.